Explosive welding of metals has been in practice commercially since the mid 1960's and involves the use of explosives to accelerate metals into each other with sufficient velocity and at an angle to allow a metallurgical joint to be created. The process is well suited to bonding flat sheets and plates together. Initially, the industry focused on creating products involving flat materials such as composite metal plates used for stamping coins and corrosive resistant metal composites for the chemical process industries.
The process of explosive welding has been adapted for the purpose of bonding coaxial tubes, but typically has been limited to using explosives to implode a larger tube onto a smaller tube. The typical tube explosively bonded using this technique is 12 to 24 inches (30.5 to 61 cm) long. Lengths of 60 inches (152.4 cm) have been achieved, but with significant difficulty and added expense. Bonding longer length tubes is problematic due primarily to the difficulty in: 1) ensuring uniform density and detonation velocity of the explosive over the entire length of the tube; and 2) preventing excessive heat from accumulating at the surfaces that are being bonded as the air is pushed out between the two tubes during the explosive event. The added heat can create resolidified melt pockets and intermetallics at the bonded joint which are typically brittle and degrade the quality of the joint. In addition, variation in the forces at the detonation front of the explosives can create wrinkles and anomalies in the imploding larger tube as it collapses onto the smaller tube.
U.S. Pat. Nos. 5,261,591 and 5,259,547, both to Hardwick, disclose methods for producing explosively welded coaxial tubes of longer lengths comprising explosively welding tubes in short lengths, followed by mechanically working the tubes into longer lengths using techniques such as extrusion and drawing. Such techniques are expensive, time-consuming and require specialized equipment.
U.S. Pat. No. 4,879,890 also to Hardwick discloses a method of explosively expanding a tubular metal component into engagement with a surrounding metal component using an insert comprising a hollow cylindrical container and shock wave transmitting liquid such as water. Likewise, U.S. Pat. No. 4,708,280 to Bement discloses a tool to insert inside a metal tube for explosively joining tubes comprising an initiator, a tool form, and one or more bands of ribbon explosive wrapped around the tool to form the joining charge. Use of inserts or tools is expensive and requires a customized tool or insert to fit the size of metal tube involved. Techniques involving inserts also are associated with the uneven transfer of energy to the tube to be bonded. In addition, use of liquid environments requires fixturing to contain the liquids and liquid between the explosive and the tube decreases the pressure on the expanding tube.
Accordingly, there is a need in the art for a uniform, reliable, consistent, and cost-efficient method of welding metal tubes to the inside of larger metal tubes, including tubes of longer lengths, that does not require an insert, tool, liquid environment, extrusion or drawing, and which provides a uniform detonation front and dissipation of heat generated at the explosive front.